This invention relates to a process for the introduction of selenium into aromatic molecules and more specifically, to a one-step process for the preparation of aromatic diselenide compounds.
Organic diselenides offer many advantages over elemental selenium in the method of manufacturing and use of electrophotographic plates and are advantageously employed as a photoconductive element in electrophotographic plates. Although aliphatic diselenides such as benzyl diselenide and polyselenide polymers are extensively used in electrophotography, the aromatic diselenides are preferred. Generally, the introduction of selenium into aliphatic and benzylic positions is accomplished by using a nucleophilic selenium reagent and by having a readily displacable group at these aliphatic or benzylic sites. For example, benzyl chloride can be reacted with sodium diselenide to form dibenzyl diselenide since the benzylic chlorine is in an activated position. Another method for the preparation of aromatic selenides involves the reaction of K.sub.2 S.sub.2 with 1-chloroanthraquinone in alcohol to form di(anthraquinonyl-1) diselenide. This reaction will go fairly rapidly in alcohol due to the electron withdrawing nature of the oxygen atoms on the ring adjacent to the halogen bearing ring of the 1-chloroanthraquinone. However, aromatic halides in which the halogen is not in an activated position do not readily react with alkali metal diselenides. Thus, the preparation of aromatic diselenides from halogented aromatic compounds in which the halogen is bonded directly to the aromatic nucleus and which does not have a strong electron withdrawing group on the same ring or the ring adjacent to the halogen bearing ring has proven problematical. In order to achieve substitution when this type of aromatic halide is employed, an extremely active leaving agent, such as the diazonium group, must be used. In an alternative method, the insertion of elemental selenium into a Grignard reagent has been employed.
Several techniques for the preparation of aromatic diselenide polymers have been developed. One of the approaches is exemplified by the synthesis of phenylene diselenide polymers as illustrated by the following reactions: ##SPC1##
One basis for introduction of selenium into the aromatic system is the well known reaction of the element with Grignard reagents. The obvious problem lies in the necessity to generate a double Grignard reagent. With the expected variability in the conversion efficiency, the air oxidation of the selenol and diselenol mixture yields a poorly defined mixture of diselenides.
Another general approach to the diselenide polymer synthesis is via intermeidate formation of phenylene bis-selenocyanates: ##SPC2##
The alkaline hydrolysis of bis-selenocyanates is, of course, one way by which the earliest examples of aromatic diselenide polymers were produced.
From the examples given, the preparation of aromatic diselenide polymers until now has proven to be quite difficult in that several synthetic steps were required which are laborious and which require a great amount of time and expense. In addition, multi-step syntheses are restrictive and often produce a poor yield of the desired compound or polymer.
It would be desirable, and it is an object of the present invention, to provide a novel one-step process for the preparation of aromatic diselenide compounds.
A further object is to provide such a process in which the aromatic diselenide compound is an aromatic diselenide polymer.
Another object is to provide such a process wherein the aromatic diselenide compounds are prepared by the reaction of an alkali metal diselenide and an aromatic halide.
A further object is to provide such a process in which the reaction will proceed when the halogen on the aromatic halide is not in an activated state.
An additional object is to provide such a process in which the aromatic halide is reacted directly with the alkali metal diselenide without having first been converted to an intermediate composition.